1. Field of the Invention
The present invention relates to an image-forming apparatus used for copiers, printers and facsimiles. More specifically, the invention relates to an image-forming apparatus capable of obtaining images of suitable densities even when the thickness of the photosensitive material is decreased due to abrasion of the film yet saving the consumption of the toner.
2. Description of the Prior Art
In general, photoattenuation characteristics of the electrophotographic photosensitive materials are dependent upon the thickness of the film. That is, the thickness of a photosensitive material and, particularly, of an organic photosensitive material decreases with an increase in the frequency of use as the film is gradually abraded, and the potential (Vr) at the exposed portion changes (generally, increases) as the thickness of the film decreases. When the photosensitive material is used for extended periods of time, therefore, the potential at the exposed portion changes to such an extent that the image is no longer properly obtained.
In the conventional image-forming apparatuses, therefore, attempts have been made to extend the life of the photosensitive material by such means as correcting the exposure amount depending upon the abrasion of the film.
However, the dependence of potential of the exposed portion of the photosensitive material upon the thickness of the film is not necessarily monotonous; e.g., some photosensitive materials exhibit the potential at the exposed portion that change linearly depending upon the thickness of the film, and some photosensitive materials exhibit the potential at the exposed portion that so changes as to possess a minimum value depending upon the thickness of the film. Therefore, means for correcting the exposure amount must be changed depending upon the properties of the photosensitive material that is used. It is, however, very difficult to bring the control means into match with the properties of the photosensitive material to a sufficient degree.
According to the conventional systems as described above, it is difficult to maintain the potential of the exposed portion to lie within a predetermined range over a wide range of the thickness of the film. Even though obtaining the favorable image can be extended to some extent, the photosensitive material must be renewed after the use of relatively short periods of time, which is not still satisfactory from the standpoint of maintenance and cost.
It is therefore an object of the present invention to provide an image-forming apparatus in which the potential of the exposed portion of the photosensitive material varies within a narrow range depending upon the thickness of the film, the photosensivitive material can be used over a wide range of the thickness of the film without substantially causing a drop in the image density without using cumbersome means such as correcting the exposure amount, and which saves the consumption of the toner as compared at a predetermined image density.
According to the present invention, there is provided an image-forming apparatus for forming an image by electrically charging a photosensitive material, forming an electrostatic image through the exposure to light and developing the electrostatic image, wherein the main charged potential (Vo) of the photosensitive material, the potential (Vr) of the exposed portion of the photosensitive material and the developing bias potential (DB) are so set as to satisfy the following formulas (1) and (2),
V1xe2x89xa6|DBxe2x88x92Vr|xe2x89xa6V2xe2x80x83xe2x80x83(1)
V3xe2x89xa6|Voxe2x88x92DB|300xe2x80x83xe2x80x83(2)
wherein V1, V2 and V3 are parameters calculated from a developing characteristics curve obtained by plotting the image densities to a potential difference between the developing bias potential and the surface potential (0 V) of the photosensitive material of when the image is formed by changing the developing bias potential with the exposure amount being zero in a state where the potential on the surface of the photosensitive material is maintained at zero,
V1 being a potential difference (absolute value) at a point at where an approximated straight line A in a region where the image density linearly increases intersects an approximated straight line B in a region where the image density increases to its saturation on the developing characteristics curve;
V2 being a potential difference (absolute value) at a point at where the approximated straight line B separates away from the developing characteristics curve; and
V3 being a potential difference (absolute value) at a point at where the image density exhibits a value of (base paper density +0.005).
In the image-forming apparatus of the present invention, it is desired that:
1. The main charging potential (Vo) of the photosensitive material and the developing bias potential (DB) are so set as to satisfy the following formula (2xe2x80x2),
V3+50xe2x89xa6|Voxe2x88x92DB|xe2x89xa6250xe2x80x83xe2x80x83(2xe2x80x2)
2. The exposure amount of the exposure device for irradiating the photosensitive material with light is set to be from 3 times to 10 times as large and, preferably, from 4 times to 6 times as large as the half exposure amount of the photosensitive material;
3. The photosensitive material is an organic photosensitive material containing a charge-generating agent and a charge-transporting agent; and
4. The developing is effected by the reversal developing system by using a two-component developing agent.